Audiences in 1950 had no trouble identifying where the somewhat ungainly title came from. It's from the fourth line of a well-beloved folk song, not heard much anymore: "Oh, dear, what can the matter be?/ Dear, dear, what can the matter be?/ Oh, dear, what can the matter be?/ Johnny's so long at the fair." It's no stretch to understand who Johnny is in the movie: It's Jean Simmon's character's missing brother, Johnny, and he certainly seems to have overstayed his welcome at this Paris World's Fair.
Based upon an urban legend popular in America and Europe.
The studio planned to dupe the public into thinking that the attractive pairing of Jean Simmons and Dirk Bogarde was mirrored in real life. This backfired suddenly however when Simmons ran off with Stewart Granger.
This same story was filmed for Season One of the TV series, Alfred Hitchcock Presents, under the title "Into Thin Air". The show starred Hitchcock's daughter, Patricia Hitchcock. The change to the plot was that the girl arrived in Paris with her mother instead of her brother.